1. Field of the Invention
This invention relates to a perpendicular-alignment type liquid crystal display device, and in particular to a liquid crystal display device in which viewing angle characteristics are improved, so that degradation of color reproduction of an image when viewed at an oblique angle with respect to the display screen is alleviated.
2. Description of the Related Art
Liquid crystal display devices can be classified into various display modes according to how the liquid crystal material is controlled when displaying text and images. For example, a twisted-nematic (TN) liquid crystal type is a display type utilizing nematic liquid crystals with a liquid crystal molecule twist angle of 90°, and is normally-white, with the liquid crystal molecules oriented in a horizontal direction and transmittance maximum in the state in which no voltage is applied; by applying a voltage the liquid crystal molecules are aligned in the perpendicular direction, and transmittance falls. However, in a TN-mode liquid crystal display device, viewing angles are generally narrow, and a viewing-angle broadening film or similar is necessary.
Problems with TN-type devices include the occurrence of a grayscale inversion phenomenon viewed from an oblique direction, at a low display luminance or brightness with a high voltage applied. In this grayscale inversion phenomenon, when the applied voltage is increased, grayscales change from a decreasing trend to a temporary increase, and then decrease once again. In order to suppress this decrease, halftone technologies have been proposed, in for example Japanese Patent Laid-open No. 3-122621, Japanese Patent Laid-open No. 4-348324, Japanese Patent Laid-open No. 5-66412, Japanese Patent Laid-open No. 5-107556, Japanese Patent Laid-open No. 6-332009, Japanese Patent Laid-open No. 8-507880, Japanese Patent Laid-open No. 7-13191, Japanese Patent Laid-open No. 7-72509, and Japanese Patent Laid-open No. 7-191634. In these halftone technologies, pixel electrodes are divided into a plurality of capacitively coupled subpixel electrodes, a single voltage is applied to the plurality of subpixel electrodes via a common data bus line and a thin film transistor, and due to the capacitive coupling, different voltages are applied to the subpixel electrodes, the applied voltage-transmittance characteristics are shifted, and the above-described grayscale inversion phenomenon is suppressed.
In these halftone technologies, a voltage is applied to a plurality of subpixels via capacitive coupling, so that there is the problem that the applied voltage needs to be too high. Also, there is also the problem that capacitance must be formed between electrodes in three dimensions for the capacitive coupling, so that structures become complicated. Also, because TN-type liquid crystal display devices are normally-white when a voltage is not applied, it is necessary to block light leaving from the gaps between subpixel electrodes, so that a light-blocking film to block light must be provided between subpixel electrodes within pixels; consequently the aperture ratio is reduced, and there is the problem that overall luminance is decreased. Due to such problems, products adopting halftone technologies for TN-mode devices have not yet been announced.
On the other hand, in-plane switching (IPS) type methods and vertical-alignment (VA) type methods and similar have also been proposed as techniques to realize broad viewing angles. Of these, VA type liquid crystal display devices modify the transmittance by moving liquid crystals aligned normally to the substrate in horizontal direction by application of a vertical electric field in the perpendicular direction. Normally-black operation, in which the transmittance is small (substantially 0%) when no voltage is applied, is possible, and by applying a voltage the transmittance can be increased to display white. Also, these applicants have proposed a multi-domain VA (MVA) type device, in which the direction of inclination of liquid crystal molecules is restricted to a plurality of directions within one pixel. See for example Japanese Patent Publication No. 2,947,350.
FIG. 1 through FIG. 3 are drawings to explain an MVA type liquid crystal display device. As shown in the cross-sectional view of FIG. 1, a thin film transistor (TFT) substrate 1 on which are formed TFTs to apply a voltage to pixel electrodes, data bus lines, gate bus lines and similar, and an opposing substrate 2 on which is formed a common electrode, are placed in opposition with spacers 3 and a liquid crystal layer 4 intervening; the periphery of the substrates 1 and 2 is sealed by a peripheral seal 5. Polarizing plates 6 and 7 are provided on both substrates and a mounting terminal group 8 for connection of driving circuitry and similar is formed on the TFT substrate.
FIG. 2 is a plan view of the TFT substrate 1; data bus lines DB1, DB2 are provided in the vertical direction with respect to the pixel electrodes PX1, PX2 arranged in a matrix, and are connected to the pixel electrodes PX1, PX2 respectively via the thin film transistors TFT1, TFT2, which are switching transistors. The gate bus line GB1, which controls conduction of the TFTs, and an auxiliary electrode bus line SE1 to suppress fluctuations in the voltage applied to the pixel electrodes, are provided in the horizontal direction.
FIG. 3 is a cross-sectional view which explains the perpendicular or vertical alignment of liquid crystal molecules between pixel electrode and the common electrode in an MVA type device. Pixel electrodes PX are formed on the TFT substrate 1, and projections 14 are provided on the surface of the pixel electrodes PX to regulate the alignment direction of liquid crystal molecules. An alignment film 10 is formed on the uppermost surface thereof. On the other hand, the common electrode COM, which is the opposing electrode, is formed on the opposing substrate 2, and this is covered with an alignment film 12 on which are provided projections 14 which regulate the alignment direction of liquid crystal molecules thereabove. By providing projections, liquid crystal molecules which are aligned in the perpendicular direction with no voltage applied can be caused to incline somewhat, as shown in the figure, according to the shape of the projections. Together with this, the direction of inclination of liquid crystal molecules when a voltage is applied can be restricted to the same direction. When projections which act as such alignment regulation means are not provided, the directions of inclination of liquid crystal molecules with a voltage applied are scattered, and singularities are formed in the display area; but by providing alignment regulation means, the direction of inclination of liquid crystal molecules can be restricted to a plurality of directions, and scattering of the direction of inclination can be prevented. Further, when liquid crystal molecules are inclined in only a single direction while a voltage is applied, a bias occurs in the viewing angle characteristics; but by providing alignment regulation means and causing the liquid crystal molecules to be inclined in a plurality of directions during voltage application, the above bias in viewing angle characteristics is averaged, and the viewing angle characteristics can be improved.
FIG. 4 is a cross-sectional view used to explain the vertical alignment of liquid crystal molecules between pixel electrodes and the common electrode in another MVA type device. In this example, the configuration of the TFT substrate 1 is the same as in FIG. 3, but on the opposing substrate 2, slits 20 are provided in the common electrode COM rather than projections as the alignment regulation means. By providing the slits 20, the direction of the electric field in the liquid crystal molecule layer can be inclined, the liquid crystal molecules are in a somewhat inclined state with respect to the electric field direction during voltage application, and in effect the same state as when projections are provided can be reproduced.
Projections have a greater alignment regulation effect, but entail a complex structure and cause increases in cost; provision of slits in the electrode results in a simpler structure and reduced costs.
In addition, it has been proposed (for example, in Japanese Patent Laid-open No. 2000-235371 and Japanese Patent Laid-open No. 2002-72985) that in an liquid crystal display device, each pixel electrode be divided into two subpixel electrodes, and voltages of opposite polarity be applied to the subpixel electrodes to prevent flicker. However, there are no descriptions of vertical alignment (VA) or of multi-domain vertical alignment (MVA) type devices, nor is there a description of problems specific to vertical alignment type devices.